(a) Field of the Invention
The present invention relates to a vertical handoff method, and more particularly, the present invention relates to a vertical handoff method of a mobile station that can access a first network having relatively wide service coverage and a second network having relatively narrow service coverage in an area where service coverages of heterogeneous networks are overlapped.
(b) Description of the Related Art
It is expected that a next generation network will evolve into an Internet protocol (IP)-based wireless network with overlapped service areas. That is, heterogeneous wireless networks interact with each other, forming a hierarchically overlapped service environment. In such a heterogeneous wireless network environment, wireless networks having difference characteristics interact with each other and their services areas are hierarchically overlapped with each other so that a service subscriber can selectively and flexibly access an optimal network according to a location of the service subscriber, a propagation condition, service characteristics, and a user preference.
Particularly, among such heterogeneous wireless networks, interaction between a third generation cellular network and a wireless local area network (WLAN) hotspot network is currently occurring.
In general, the third generation cellular network provides global coverage but it has a low data rate with high cost. On the other hand, the WLAN hotspot network provides a limited coverage but it has a high data rate with low cost.
Therefore, seamless service can be guaranteed by informing switching in service connection paths for interaction between the third generation cellular network and the WLAN hotspot network, each having different characteristics, to the mobile station, and accordingly, an optimal service can be provided to a subscriber.
Herein, a handoff that occurs between heterogeneous wireless networks such as between the third generation cellular network and the WLAN hotspot network is called a vertical handoff or an inter-system handoff. The vertical handoff is different from a horizontal handoff that occurs between cells in homogeneous wireless networks.
The vertical handoff uses a signal strength value for a handoff decision, and the signal strength of the vertical handoff is different from that of the horizontal handoff. The horizontal handoff is decided by comparing the corresponding signal strength values of base stations. However, the vertical handoff cannot be decided by comparing signal strengths because there are no signal strengths that can be compared. Since the third generation cellular network has global service coverage, signal strength cannot be used as triggering information for handoff decision. Therefore, the vertical handoff is decided by using a signal strength of a network (e.g., WLAN hotspot network) having a limited service coverage.
In addition, the vertical handoff has a higher probability of occurrence of the ping-pong effect compared to the vertical handoff.
That is, the horizontal handoff can reduce the ping-pong effect by using a signal strength difference between neighboring base stations, a timer, and a hysterisis effect. However, since the vertical handoff has no triggering information that can be compared except for a signal strength of the WLAN hotspot network that provides the limited service coverage, the vertical handoff has the higher probability of occurrence of the ping-pong effect.
FIG. 1 shows the ping-pong effect that occurs in realization of a conventional vertical handoff method.
As shown in FIG. 1, one vertical handoff threshold value is set.
When there is set one vertical handoff threshold value Xthresh, a signal strength received at a mobile station located in a border area of the WLAN hotspot network is fluctuated in a small range that is close to the vertical threshold value Xthresh, causing occurrence of the ping-pong effect.
In addition, the mobile station in the middle of performing a vertical handoff to the WLAN hotspot network may be forced to perform a vertical handoff to the third generation cellular network due to the limited coverage of the WLAN hotspot network. Such a phenomenon may increase a failure probability of the vertical handoff.
However, the ping-pong effect in the vertical handoff may give a heavier load to a system compared to the horizontal handoff due to characteristics of the vertical handoff, and therefore it is highly required to minimize the ping-pong effect.
In addition, it is more beneficial for a subscriber to stay longer in the WLAN hotspot network providing the high data rate with low cost, and therefore, a method for supporting the subscriber to stay in the WLAN hotspot network for a longer time period is required.
In order to minimize the ping-pong effect, the following two prior arts have been proposed.
According to one of the two prior arts, a mobile station selects a network that provides optimal performance by using a cost function. In addition, the mobile station monitors whether the selected network provides consistent performance during a stability period Ts and evaluates the monitoring result, and then performs a vertical handoff to the selected network.
Herein, the stability period Ts equals Ihandoff+Tmakeup. In this case, Ihandoff denotes a vertical handoff delay time and Tmakeup denotes a time taken for compensating a loss due to the vertical handoff.
In addition, the monitoring and evaluation of the performance of the selected network may vary depending on a variation of a valid bandwidth and a variation of a received signal strength (RSS) due to mobility of the mobile station.
Therefore, when the mobile station performs the vertical handoff when the mobile station can provide constant performance in a target wireless network during the stability period Ts, that is, when it is expected that the variation of the valid bandwidth due to the variation of the received signal strength is not reduced lower than a reference value, the vertical handoff is performed.
However, although the target wireless network guarantees the constant performance of the mobile station during the stability period Ts, irregular mobility of the mobile station may cause inconsistent performance.
Accordingly, this prior art cannot efficiently prevent the ping-pong effect.
According to another prior art, the mobile station monitors whether a received signal strength received from the WLAN hotspot network is greater or less than the vertical handoff threshold value Xthresh within a transition region t. After monitoring λn sequentially detected received signal strengths, the mobile station decides whether to perform the vertical handoff.
However, the received signal strength of the mobile station cannot be linear due to irregular characteristics of the wireless network environment. Therefore, although values of the λn received signal strengths exceed the vertical handoff threshold value Xthresh, there may exist a received signal strength that is lower than the vertical handoff threshold value Xthresh.
In this case, the mobile station does not perform the vertical handoff and repeats the monitoring of λn received signal strengths. That is, the mobile station repeats a vertical handoff from the third generation cellular network to the WLAN hotspot network and from the WLAN hotspot network to the third generation cellular network.
Due to such a ping-pong effect, the mobile station cannot find an appropriate time to perform the vertical handoff, and therefore the failure probability of the vertical handoff is increased, causing performance deterioration.
Moreover, conventionally, reduction of one received signal strength among received signal strengths received during a predetermined time period causes the mobile station to not perform a vertical handoff.
However, a received signal strength received from the WLAN hotspot can be temporarily significantly decreased depending on the wireless environment state. Thus, when handoff decision is made by one received signal strength, the vertical handoff decision may not be made even though a strong enough received signal strength is received from the WLAN hotspot network.
However, as described, a received signal strength received from the WLAN hotspot network can be temporarily significantly decreased. Thus, when the handoff decision is made by one received signal strength, the vertical handoff decision may not be made even though a weak received signal strength is received from the WLAN hotspot network. The above phenomenon occurs because a substantial received signal strength received at the mobile station is not in a linear form.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.